1. Field of the Invention
The present invention relates generally to a voltage-controlled oscillator (VCO), and more particularly, to a quadrature VCO and a method of providing quadrature output signals.
2. Description of the Related Art
A VCO has been indispensable in the operations of fully integrated wireless receiver architectures including superheterodyne, direct-conversion, and low-IF (intermediate frequency). The functionality of purposed VCO is to provide a signal mixed with the RF (radio frequency) signals and to downconvert the RF signal into an intermediate frequency via a mixer. To eliminate an image frequency, a quadrature-conversion method is usually adopted for mixing the RF signal via I/Q quadrature paths to result in two inverse spectrums to achieve the goal of image frequency elimination. A local oscillation signal is also provided by the VCO, and moreover, the output signal is injected into a quadrature signal generator to induce a quadrature-phase output. Furthermore, it will not be any quadrature signal generator as the VCO can output the I/Q quadrature signals, and simultaneously, the advantage is to reduce both the chip area and cost effectively. Generally, a quadrature VCO does not have outstanding performance in phase noise and power consumption, and several conventional QVCOs with different coupling mechanism have been presented as below. As shown in FIG. 4 showing the first conventional architecture, a quadrature VCO is composed of two resonator units 30, each of which includes two NMOSs (N-channel metal oxide semiconductor). Each of the NMOSs of one of the resonator 30 is a cross-coupled pair 31 and those of the other resonator 30 are coupled transistors 33. An LC (inductor-capacitor) resonator unit 35 is connected in parallel with the cross-coupled pair 31 to constitute an oscillator that can keep oscillation. However, each of the resonator units 30 needs two different constant current sources 36 and 37 to reach the minimum bias current and enable the oscillator to start oscillation, so the power consumption is increased. The phase noises of the resonator units are also inverse proportion of the bias current, so the smaller the bias current is, the higher the noise will be, thus deteriorating the circuit performance. In addition, the coupled transistors 33 in parallel can also generate noises for the output terminal to make the phase noise increased.
As shown in FIG. 5 showing a second conventional architecture, another quadrature VCO is constituted by superimposition of a coupled transistor 43 and a cross-coupled pair. The advantage is to reduce the noise generated by the coupled transistor 43 for the output terminal, and that the structure of the quadrature VCO has less phase noise than previous. Additionally, the superimposition allows the bias current to be reused to decrease the power consumption where the structure of quadrature VCO is called top-series. However, the drawback is that the noise is still high.
As shown in FIG. 6 showing third conventional architecture, another quadrature VCO is also constituted by superimposition of a coupled transistor 53 and a cross-coupled pair 51, which is similar to the superimposition of the VCO of the second architecture as long as the coupled transistor is exchanged with the cross-coupled pair. In this way, the noise generated by the transistors of the cross-coupled pair 51 for the output terminal can be reduced, and the structure of quadrature VCO is called bottom-series. The advantage of this conventional quadrature VCO has lower phase noise, but it has induced the higher phase error.
To sum up, the phase error of the top series is superior to that of the bottom series, but the phase noise of the bottom series is superior to that of the top series.
As shown in FIG. 7 showing a fourth conventional architecture, another quadrature VCO is to generate quadrature output signals by common-mode inductive coupling. The transformer base is used to couple the output signals; however, the phase difference of two oscillators keeps set at 180 degree by second harmonic mechanism. The drawback of this structure is using the transformer component will increase the chip area and chip cost, so the quadrature signals are generated, thus increasing the chip area to heighten the cost.